CN117916019A

CN117916019A - Catalyst article for exhaust system of natural gas engine

Info

Publication number: CN117916019A
Application number: CN202280061381.7A
Authority: CN
Inventors: M·H·楚玛; G·琼斯; J·N·穆戈; A·拉杰
Original assignee: Johnson Matthey PLC
Current assignee: Johnson Matthey PLC
Priority date: 2021-10-12
Filing date: 2022-10-06
Publication date: 2024-04-19
Also published as: US20230110753A1; TW202325387A; US11980871B2; EP4166230A1; KR20240042055A; GB202214666D0; WO2023062340A1; GB2613243A

Abstract

The present invention relates to catalyst articles for use in exhaust systems of natural gas engines having improved sulfur and/or water resistance. The catalyst article comprises a doped palladium-alumina catalyst, wherein the palladium-alumina catalyst is doped with manganese and/or zinc. The invention also relates to an exhaust gas treatment system, a natural gas combustion engine and a method for treating exhaust gas from a natural gas combustion engine.

Description

Catalyst article for exhaust system of natural gas engine

The present invention relates to a catalyst article for an exhaust system of a natural gas engine, in particular to a catalyst article having improved sulfur and water resistance due to the presence of additional Mn and/or Zn in the Pd alumina catalyst.

Natural gas is of increasing interest as an alternative fuel to vehicles and stationary engines traditionally using gasoline and diesel fuel. Natural gas consists mainly of methane (typically 70-90%) with variable proportions of other hydrocarbons such as ethane, propane and butane (up to 20% in some deposits) and other gases. Natural gas can be commercially produced from oil or gas fields and is widely used as a combustion energy source for power generation, industrial cogeneration, and home heating. It can also be used as a vehicle fuel.

Natural gas can be used as a transportation fuel in the form of Compressed Natural Gas (CNG) and Liquefied Natural Gas (LNG). CNG is contained in a tank at a pressure of 3600 pounds per square inch (-248 bar) and has an energy density per unit volume of about 35% of gasoline. LNG has an energy density 2.5 times that of compressed natural gas and is mainly used in heavy vehicles. It cools to a liquid state at-162 c, thus reducing the volume by a factor of 600, which means that LNG is easier to transport than CNG. Biological liquefied natural gas can be a substitute for natural gas (fossil) produced from biogas produced by anaerobically digesting organic matter such as landfill waste or manure.

Natural gas has many environmental benefits: it is a cleaner burning fuel that generally contains few impurities, its energy per carbon (Bti) is higher than conventional hydrocarbon fuels, so carbon dioxide emissions are low (25% reduction in greenhouse gas emissions), and it has lower PM and NO _x emissions compared to diesel and gasoline. Biogas can further reduce this emission.

Further driving factors for the adoption of natural gas include high abundance and low cost compared to other fossil fuels.

Natural gas engines emit very low PM and NO _x (as low as 95% and 70%, respectively) compared to heavy and light duty diesel engines. However, the exhaust gas produced by NG engines typically contains a significant amount of methane (so-called "methane leakage"). Regulations limiting these engine emissions currently include european VI and the united states Environmental Protection Agency (EPA) greenhouse gas regulations. These specify emission limits for methane, nitrogen oxides (NOx), and Particulate Matter (PM).

Two main modes of operation for methane-fuelled engines are stoichiometric (λ=1) and lean (λ+.1.3). Palladium-based catalysts are known to be the most active type of catalyst for methane oxidation under two conditions. By applying a palladium-rhodium three-way catalyst (TWC) or a platinum-palladium oxidation catalyst, respectively, the prescribed emission limits of both stoichiometric and lean burn compressed natural gas engines may be met.

The development of such palladium-based catalyst technology is dependent on challenges in overcoming the cost and catalyst deactivation due to sulfur, water and thermal aging.

Methane is the least reactive hydrocarbon and requires high energy to break the primary C-H bonds. The ignition temperature of alkanes generally decreases with increasing fuel-air ratio and increasing hydrocarbon chain length, which is related to the c—h bond strength. It is well known that for Pd-based catalysts, the light-off temperature for methane conversion is higher than for other hydrocarbons (where "light-off temperature" refers to the temperature at which the conversion reaches 50%).

TWCs are used as efficient and cost-effective aftertreatment systems for combusting methane when operated under stoichiometric conditions (λ=1). Most bimetallic Pd-Rh catalysts have a high total platinum group metal (pgm) loading of >200gft ^–3, which is required for high levels of methane conversion to meet the regulations for end-of-life Total Hydrocarbons (THC) because the reactivity of such hydrocarbons is very low and the catalyst is deactivated by thermal and chemical effects. The use of high pgm loading will increase the total HC conversion in the stoichiometric CNG engine. However, based on engine calibration, high methane conversion may be achieved with relatively low pgm, i.e., controlling the air-to-fuel ratio to operate near or rich of stoichiometry; the pgm loading can also vary according to regional legislation requirements regarding methane and non-methane conversion.

The reduction of NO _x and the oxidation of methane are also more difficult under very oxidizing conditions. For lean CNG applications, high total pgm loadings (> 200gft ^–3) of Pd-Pt are required to perform methane combustion at lower temperatures. Unlike stoichiometric engines, it is also desirable to inject a reductant into the exhaust stream to be able to reduce NO _x in the presence of excess oxygen. This is typically in the form of ammonia (NH ₃), so lean burn applications require a completely different catalyst system than stoichiometric, where CO or HC can be used under slightly rich or stoichiometric conditions to achieve efficient NO _x reduction.

Due to the non-reactivity (or poor reactivity) of methane at lower temperatures, methane emissions increase during cold start and idle conditions, primarily at exhaust temperatures below stoichiometric lean conditions. In order to increase the reactivity of methane at lower temperatures, one option is to use high pgm loadings, which increases costs.

Natural gas catalysts, especially Pd-based catalysts, may be poisoned by water (5-12%) and sulfur (SO ₂ <0.5ppm in lubricating oils), especially under lean conditions, which can lead to a dramatic decrease in the conversion of the catalyst over time. Deactivation by water is significant due to the formation of hydroxyl, carbonate, formate and other intermediates at the catalyst surface. This activity is reversible and can be fully restored if water is removed. However, this is not practical because methane combustion feeds always contain high levels of water due to the high content of H in methane.

H ₂ O may be an inhibitor or accelerator depending on the air-fuel ratio, i.e., lambda. Under stoichiometric and reducing conditions, lambda >1, h ₂ O can act as an accelerator for hydrocarbon oxidation by steam reforming reactions in both CNG and gasoline engines. However, for lean CNG operating at λ >1, H ₂ O acts as a methane oxidation inhibitor. It is important to understand the inhibition of water and to design a catalyst that is more tolerant to the presence of H ₂ O. This would allow improvement when trying to control methane emissions from lean-burn CNG.

Despite the very low sulfur content in engine exhaust, pd-based catalysts can significantly deactivate after exposure to sulfur due to the formation of stable sulfates. Regeneration of the catalyst to restore activity after sulfur poisoning is challenging and typically requires high temperatures, rich operation, or both. This is easily achieved in stoichiometric operation, but more difficult to achieve in lean burn. Lean-burn vehicles operate at a much higher air-fuel ratio than stoichiometric vehicles and will require injection of a much higher concentration of reductant to switch to rich operation. Thermal deactivation due to high-level misfire events caused by engine transient control and poor ignition systems destroys the catalyst and correspondingly leads to high levels of exhaust emissions.

The catalyst deactivates under both conditions, but sulfur poisoning has a more significant impact than thermal aging in lean operation. Sulfur poisoning can be ameliorated by adding small amounts of Pt to the Pd catalyst. This is because sulfur inhibition due to the formation of palladium sulfate can be significantly reduced when Pt is added. However, the addition of Pt further increases the cost.

Accordingly, it would be desirable to provide an improved catalyst for a natural gas combustion engine to reduce methane emissions by inhibiting catalyst deactivation (such as by sulfur, water, and thermal aging) without increasing the cost of the catalyst. It is an object of the present invention to address this problem, to address the disadvantages associated with the prior art, or to at least provide a commercially useful alternative.

According to a first aspect, there is provided a catalyst article for treating exhaust gas from a natural gas combustion engine, the article comprising a doped palladium-alumina catalyst, wherein the palladium-alumina catalyst is doped with manganese (Mn) and/or zinc (Zn).

In the following paragraphs, different aspects/embodiments are defined in more detail. Each aspect/embodiment so defined may be combined with any other aspect/embodiment or aspects/embodiments unless clearly indicated to the contrary. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature or features indicated as being preferred or advantageous.

Catalyst articles are suitable components for use in exhaust systems. Typically, such articles are honeycomb monoliths, which may also be referred to as "bricks". These have a high surface area configuration suitable for contacting the gas to be treated with a catalyst material to effect conversion or conversion of exhaust gas components. Other forms of catalyst articles are known and include plate constructions and wrapped metal catalyst substrates. The catalyst described herein is suitable for all of these known forms, but it is particularly preferred that it takes the form of a honeycomb monolith, as these catalysts provide a good balance of cost and manufacturing simplicity.

Preferably, the doped palladium-alumina catalyst is provided as a washcoat on the substrate. Or a doped palladium-alumina catalyst is provided as a component of the extruded substrate. Preferably, in either case, the substrate is a flow-through monolith. Preferably, wherein the catalyst is provided as a washcoat on a substrate, the substrate being a flow-through ceramic monolith.

The catalyst article is for treating exhaust gas from a natural gas combustion engine. That is, the catalyst article is used to catalytically treat exhaust gas from a natural gas combustion engine to convert or convert the gas components before the gas is emitted into the atmosphere in order to meet emission regulations. When natural gas burns, it produces carbon dioxide and water, but the exhaust also contains some additional methane (and other short-chain hydrocarbons) that needs to be catalytically removed before the exhaust is discharged to the atmosphere. The exhaust gas also typically contains significant amounts of water and sulfur that can accumulate and deactivate the catalyst.

In mobile applications, natural gas combustion may be configured to operate in a lean or stoichiometric configuration. By "mobile application" is meant that the system is generally adaptable to automobiles or other vehicles (e.g., off-road vehicles) -in such systems, fuel supply and demand may vary during operation, depending on operator demand (such as acceleration). In mobile applications, the system can typically be operated temporarily in a rich mode, which is associated with a significant increase in temperature, which helps burn off sulfur poisoning the catalyst and remove accumulated water.

In stationary systems, natural gas combustion may also be configured to operate under lean or stoichiometric conditions. Examples of stationary systems include gas turbines and power generation systems-in such systems, combustion conditions and fuel composition are typically kept constant over long operating times. This means that there is less chance of having a regeneration step to remove sulfur and moisture contaminants than in mobile applications. Thus, the benefits described herein may be particularly beneficial for stationary applications. That is, when the opportunity to regenerate the catalyst is limited, it is particularly desirable to provide a catalyst having high sulfur and moisture resistance.

While the "lean" and "stoichiometric" systems described above are described as "mobile" and "fixed," it should be understood that both system types may be used in a range of different applications.

The catalyst article comprises a doped palladium-alumina catalyst. That is, the article comprises a catalyst comprising as components (preferably as the sole catalytically active component) an alumina support provided as a palladium-supporting support and a dopant. Alumina is a very common support for catalyst applications, and the selection of suitable alumina is common in the art. As discussed herein, the appropriate amount of alumina selected for forming the catalyst will depend on the form of the substrate.

The alumina may be provided in one or more different crystalline forms. Gamma alumina is generally most preferred due to its thermal stability. The alumina may contain one or more dopants to help improve its stability, and typical dopants include Si and La. The dopant for stabilizing the alumina is preferably present in an amount of less than 15wt%, more preferably less than 10wt% of the weight of the alumina. These dopants for alumina may be present in an amount of 1 to 10wt% based on the weight of alumina.

The catalyst may also comprise additional non-catalytic components depending on the form of the catalyst article. For example, the washcoat typically includes a binder that includes clay and other alumina components. For example, extruded catalyst monoliths typically include additional fillers and processing aids such as glass fibers and clays.

Preferably, the catalyst article has a Pd loading of from 50 to 300g/ft ³, preferably from 70 to 250g/ft ³, and more preferably from 100 to 200g/ft ³. These levels are effective for treating methane in the exhaust and may be significantly higher than the level of the TWC.

The palladium-alumina catalyst is doped with manganese and/or zinc. Most preferably the catalyst is doped with Mn or Zn. Preferably, the total loading of Mn and/or Zn of the catalyst article is from 5 to 100g/ft ³, preferably from 20 to 80g/ft ³, and more preferably from 40 to 60g/ft ³. The total loading refers to the sum of all Mn and Zn present (e.g., the amount of Mn present when Zn is not present). As explained below, both of these elements have been found to be particularly effective in promoting palladium and providing improved performance of the catalyst when challenged with sulfur and/or moisture.

Preferably, the ratio of Pd loading to the total loading of Mn and/or Zn of the catalyst article is greater than 1:1, preferably from 1:1 to 10:1, more preferably from 2:1 to 5:1, and most preferably about 3:1. Mn and/or Zn are present in order to promote the reaction effected by Pd, and therefore the amount of dopant is generally equal to or less than the amount of Pd promoted.

The present inventors have sought to provide a natural gas engine aftertreatment system capable of combusting methane at lower temperatures. The inventors have surprisingly found that the inclusion of Mn and/or Zn has a positive effect on (i) the methane conversion performance under wet and dry conditions and (ii) the sulfur tolerance of the catalyst. In particular, experimental SCAT analysis showed that the Mn and Zn doped Pd/Al ₂O₃ catalysts had improved light-off activity for methane at low temperature, dry and wet conditions. This shows that doping Pd/Al ₂O₃ with Mn or Zn improves the performance of the catalyst better than the Pt-doped Pd/Al ₂O₃ catalyst. Sulfur tolerance tests were also performed on these catalysts, with both Mn and Zn doped catalysts exhibiting similar improvements in sulfur tolerance.

Without wishing to be bound by theory, doping Pd with Mn appears to reduce the activation energy (E _a) better than other dopants and to enhance its resistance to OH poisoning. Similar properties were observed with Zn.

Using computational simulations, models were developed that help understand the underlying principles of these reactions. In this model, the PdO (100) surface is doped with a series of +2 oxidation state elements as listed in table 1. The activation energy barrier (E _a) of methane on the doping surface and the adsorption energy of various intermediate substances are calculated, and a model is built. The model predicts that doping with Mn or Zn reduces activation energy and can enhance resistance to OH or SO ₂ poisoning.

TABLE 1 adsorption energies for various doped PdO (100) surfaces, activation energy barriers, OH, O and SO ₂ species.

To demonstrate the activity of these doped PdO (100) surfaces, kinetic models were then built using three variables, δg (CH ₄ _ts), δg (OH), and δg (O). Where δg (ch4—ts) is the first dissociation step of CH4, considered as the rate-determining step, δg (OH) represents the tolerance to water poisoning, and δg (O) is the adsorption of oxygen on the Pd surface to create active sites. Mn is believed to enhance the activity of the PdO (100) surface at lower temperatures.

Preferably, the doped palladium-alumina catalyst further comprises platinum, preferably at a loading of 50 to 300g/ft ³, preferably 70 to 250g/ft ³, and more preferably 100 to 200g/ft ³. Preferably, the weight ratio of Pt to Pd is less than 1:1, preferably from 1:2 to 1:10. Pt is a well known complementary pgm that is present to improve the oxidation performance of the catalyst as a whole.

The table above clearly shows that Mn and Zn are in all cases better than PdO alone. However, the other elements shown are a balance of advantages and disadvantages. Preferably, the doped palladium-alumina catalyst comprises one or more additional doping elements selected from Se, cu, cd, ge, ba, sr and Sn. As shown in the above table, each of these dopants has a different balancing effect on activation energy, OH, O, and SO ₂ absorbed energy. This means that depending on the particular application required, it may be desirable to add components with particular benefits, even if offset by some drawbacks. For example, cd has a great improvement in water resistance, while it has only a slight disadvantage in activation energy.

Preferably, the one or more additional dopant elements are present at a loading of 5 to 100g/ft ³, preferably 20 to 80g/ft ³, and more preferably 40 to 60g/ft ³. Preferably, the one or more additional dopants are present in a mass ratio of less than 1:1, preferably less than 3:1, and more preferably less than 5:1, relative to the total amount of Mn and Zn. This is because Mn and Zn generally have a positive effect, while additional dopants are generally provided to obtain secondary benefits.

According to another aspect, there is provided an exhaust treatment system comprising a catalyst article as described herein. An exhaust treatment system typically has an inlet end configured to receive exhaust from a combustion chamber and an outlet for releasing the treated exhaust into the atmosphere. Consistent with catalyst articles in an exhaust system, there may be one or more other catalytic or filtering components suitable for treating other components of the exhaust or providing additional treatment for methane to avoid leakage.

According to another aspect, there is provided a natural gas combustion engine comprising an exhaust treatment system as described herein. Preferably, the natural gas combustion engine is configured to operate in lean conditions. The natural gas combustion engine may be a stationary engine. As mentioned above, the stability of the catalyst to sulfur and moisture contamination is particularly critical because catalyst regeneration is more difficult in stationary systems. Otherwise, the catalyst would need to be regenerated off-line and separately, which can lead to process inefficiency and cost.

According to another aspect, a method for treating exhaust gas from a natural gas combustion engine is provided, the method comprising contacting the exhaust gas with a catalyst article as described herein. Preferably, the exhaust gas is obtained by combusting natural gas, the exhaust gas comprising at least 0.5ppm sulfur dioxide. Preferably, the exhaust gas is obtained by combusting natural gas, the exhaust gas comprising 5 to 12wt% water. It should be appreciated that these types of exhaust gases are conventional when treating exhaust gases from natural gas combustion, and it is these exhaust gases that most significantly benefit from the catalyst article described herein having improved water and sulfur resistance.

Preferably, during the step of contacting the exhaust gas with the catalyst article, the temperature of the exhaust gas is below 550 ℃, preferably below 500 ℃. For example, the exhaust gas may have a temperature of about 450 ℃. These conditions are particularly common in lean operating systems.

Drawings

The invention will now be further discussed in connection with the following non-limiting drawings, in which:

Figure 1 shows methane conversion (%) as a function of temperature under dry and wet conditions. In the figure, at 50% conversion, the lines are Zn doped (dry), mn doped (dry), undoped (dry), pt doped (dry), then Zn doped (wet), mn doped (wet), undoped (wet) and Pt doped (wet), from left to right.

FIG. 2 shows methane conversion (%) as a function of temperature using 0.5ppm SO ₂ reactant gas feed. In this figure, the best conversion is obtained with Mn and then Zn at 425 ℃.

Examples

The invention will now be further described in connection with the following non-limiting examples.

Example 1

The catalyst composition (SCFA 140) comprising 1wt% mn-2.85wt% pd/on undoped alumina was prepared as follows.

0.9G of manganese nitrate tetrahydrate was dissolved in a minimum amount of water and added to 3.75g of palladium nitrate, followed by further dilution with about 1ml of water. The mixture was added dropwise to 20g of alumina carrier with constant stirring, and then rinsed. The mixture was dried in an oven for 3 hours and then calcined at 500 ℃ for 2 hours.

The total loading of Pd in the catalyst was about 128g/ft ³ and the total loading of Mn was about 45g/ft ³.

Example 2

Another catalyst composition comprising 1wt% zn-2.85wt% pd on undoped alumina (SCFA 140) was prepared as follows.

0.9G of zinc nitrate hexahydrate was dissolved in a minimum amount of water and added to 3.77g of palladium nitrate, followed by further dilution with about 1ml of water. The mixture was added dropwise to 20g of alumina carrier with constant stirring, and then rinsed. The mixture was dried in an oven for 3 hours and then calcined at 500 ℃ for 2 hours.

The total loading of Pd in the catalyst was about 128g/ft ³ and the total loading of Zn was about 45g/ft ³.

Catalyst test

Granulated samples (0.2-0.4 g,250-300 μm) of the catalyst compositions prepared in examples 1 and 2 were tested for water resistance and sulfur resistance in a Synthetic Catalytic Activity Test (SCAT) apparatus using the following inlet gas mixtures with a Space Velocity (SV) of 45k in the temperature range (rising from 150 ℃ to 450 ℃ at a rising rate of 10-15 ℃/min).

For the water resistance test, the following inlet gas mixtures were used:

And (3) drying: 4000ppm CH ₄,8％O₂, balance N ₂

Wet: 4000ppm CH ₄,8％O₂,10％H₂ O, balance N ₂

For the sulfur tolerance test, the following inlet gas mixture ：4000ppm CH₄、30ppm C₃H₈、100ppm C₂H₆、1000ppm CO、5％CO₂、500ppm NO、8％O₂、10％H₂O、0.5ppm SO₂、 balance N ₂ was used.

As shown, both examples show improved sulfur tolerance, improved moisture resistance and better catalytic activity compared to undoped Pd catalysts.

Although preferred embodiments of the present disclosure have been described in detail herein, those skilled in the art will appreciate that various modifications may be made to the disclosure without departing from the scope of the invention or the appended claims.

Claims

1. A catalyst article for treating exhaust gas from a natural gas combustion engine, the catalyst article comprising a doped palladium-alumina catalyst, wherein the palladium-alumina catalyst is doped with manganese and/or zinc.

2. The catalyst article of claim 1, wherein the catalyst article has a Pd loading of 50-300g/ft ³, preferably 70-250g/ft ³, and more preferably 100-200g/ft ³.

3. The catalyst article of claim 1 or claim 2, wherein the catalyst article has a total loading of Mn and/or Zn of from 5 to 100g/ft ³, preferably from 20 to 80g/ft ³, and more preferably from 40 to 60g/ft ³.

4. The catalyst article of any one of the preceding claims, where the ratio of the Pd loading to the total loading of Mn and/or Zn is greater than 1:1, preferably 1:1 to 10:1, more preferably 2:1 to 5:1, and most preferably about 3:1.

5. The catalyst article of any one of the preceding claims, where the doped palladium-alumina catalyst is provided as a washcoat on a substrate.

6. The catalyst article of any one of claims 1 to 4, wherein the doped palladium-alumina catalyst is provided as a component of an extruded substrate.

7. The catalyst article of any one of the preceding claims, where the substrate is a flow-through ceramic monolith.

8. The catalyst article of any preceding claim, wherein the doped palladium-alumina catalyst further comprises platinum, preferably at a loading of 50 to 300g/ft ³, preferably 70 to 250g/ft ³, and more preferably 100 to 200g/ft ³.

9. Catalyst article according to claim 8, wherein the weight ratio of Pt to Pd is less than 1:1, preferably from 1:2 to 1:10.

10. The catalyst article of any one of the preceding claims, where the doped palladium-alumina catalyst comprises one or more additional dopant elements selected from Se, cu, cd, ge, ba, sr and Sn.

11. The catalyst article according to claim 10, wherein the one or more additional dopant elements are present at a loading of 5-100g/ft ³, preferably 20-80g/ft ³, and more preferably 40-60g/ft ³.

12. The catalyst article of claim 10 or claim 11, wherein the one or more additional dopants are present in a mass ratio of less than 1:1, preferably less than 3:1, and more preferably less than 5:1, relative to the total amount of Mn and/or Zn.

13. An exhaust treatment system comprising the catalyst article of any of the preceding claims.

14. A natural gas combustion engine comprising an exhaust gas treatment system according to claim 13, preferably wherein the natural gas combustion engine is configured to operate in lean conditions.

15. A method for treating exhaust gas from a natural gas combustion engine, the method comprising contacting the exhaust gas with the catalyst article of any one of claims 1-12.

16. The method of claim 15, wherein the exhaust gas is obtained by combusting natural gas, the exhaust gas comprising at least 0.5ppm sulfur dioxide and/or 5-12wt% water.

17. The method according to claim 15 or claim 16, wherein the temperature of the exhaust gas is below 550 ℃, preferably below 500 ℃, during the step of contacting the exhaust gas with the catalyst article.